# How long until the ball hits the ground?

1. Aug 25, 2011

### hyde2042

1. The problem statement, all variables and given/known data

A ball is thrown downward from a height of 30m at 8m/s. How long until it hits the ground?
(Air resistance is negligible and gravity is 9.8m/s

2. Relevant equations
I tried deltaX=Vi+(1/2)a(t^2) and came out with 2.12, but the solution says it's 1.79.

3. The attempt at a solution

30=8+(1/2)(9.8)(t^2)
22=4.9(t^2)
sqrt4.49=sqrt(t^2)
t=2.12

2. Aug 25, 2011

### CompuChip

Are you not missing a t?
$$V = V_i + a t$$
so
$$\Delta x = V_i \mathbb{t} + \frac12 a t^2$$

3. Aug 25, 2011

### hyde2042

Thank you, but I'm afraid I'm evenb more confused by the additional t.

I ended up at 30-8t=4.9(t^2)

What I keep doing after that seems to make it more complicated than it should be. COuld you possibly point me in the right direction? Thank you for your time.

4. Aug 25, 2011

### CompuChip

It's a standard quadratic equation, of the form ax2 + bx + c = 0.
How do you solve one of those?

5. Aug 25, 2011

### hyde2042

Ah... hate when I over look simple things. Thank you very much.